a) Field of the Invention
The present invention relates to a zoom lens system of the so-called negative front lens type which comprises a negative lens unit on the object side.
b) Description of the Prior Art
In the conventional negative front lens type zoom lens system in which a lens unit disposed on the object side therein or a first lens unit has negative refractive power, the role of compensator which corrects deviation of image location caused due to the vari-focal function performed by moving lens units disposed after the first lens unit is imparted to the first lens unit. In addition to the role of the compensator described above, another role of a focusing lens unit is allocated to the negative first lens unit in the conventional negative front lens type of zoom lens system. In other words, the first lens unit is moved not only for correcting the deviation of the image location caused by zooming but also for focusing the zoom lens system.
The zoom lens system of the negative front lens type described above allows remarkable variation of spherical aberration to be produced at a tele position thereof by focusing and allows optical performance thereof to be degraded by focusing the zoom lens system on an object located at a short distance. The zoom lens system of this type is unusable for photographing an object located at a shorter distance because the first lens unit has the negative refractive power, a light bundle which emerges from the first lens unit to be incident on a second lens unit is divergent, and aberrations are varied by varying an airspace reserved between the first lens unit and the second lens unit. The aberrations are varied at high degrees at the tele position of the zoom lens system at which the light bundle has a larger diameter and the zoom lens system has a larger aperture.
Further, the negative front lens type zoom lens system is undesirable for making a lens barrel thereof waterproof since a total length of the zoom lens system is varied not only by zooming but also by focusing. That is to say, the movable lens unit which is disposed on the object side in the zoom lens system makes it impossible to make the lens barrel airtight, and internal pressure of the zoom lens system and driving force of the lens barrel are varied undesirably by moving the movable lens unit.
Furthermore, the moving of the first lens unit (the lens unit disposed on the object side in the zoom lens system) for zooming causes delicate variations of moving distance of the lens unit for focusing dependently on focal lenghts of the zoom lens system and undesirably accelerates non-linearization of moving locus of the first lens unit.
The negative front lens type zoom lens system generally has a composition which is illustrated in FIG. 1 or FIG. 2. In these drawings, the reference symbol W represents a wide position, the reference symbol S designates an intermediate focal length and the reference symbol T denotes a tele position of the zoom lens system. The zoom lens system which has the composition illustrated in FIG. 1 consists of a first lens unit I and a second lens unit II. Almost all of zoom lens systems having the composition illustrated in FIG. 1 are so-called standard zoom lens systems and have zooming ratios of 2 or lower. There is also known a zoom lens system which additionally comprises a third lens unit as a flatener on the image side of the lens system shown in FIG. 1. Further, the zoom lens system which has the composition illustrated in FIG. 2 is a combination of two zoom lens systems shown in FIG. 1, and consists of a first lens unit I having negative refractive power, a second lens unit II having positive refractive power, a third lens unit III having negative refractive power power and a fourth lens unit IV having positive refractive power. Total lengths of these zoom lens systems are varied remarkably since the first lens unit is moved for a long distance as is apparent from FIG. 1 and FIG. 2.
Known as the conventional examples of the zoom lens system of this type are those disclosed by U.S. Pat. No. 4,099,845, U.S. Pat. No. 4,759,617, U.S. Pat. No. 4,516,839, U.S. Pat. No. 3,549,235 and U.S. Pat. No. 3,682,534.
Out of the conventional examples mentioned above, the zoom lens system disclosed by U.S. Pat. No. 4,099,845 consists of a front lens part which is kept fixed for zooming and has positive refractive power, a variator which is movable for zooming and has negative refractive power, a compensator which is movable for zooming and has positive refractive power, and a stationary image forming lens group which is kept fixed for zooming. This zoom lens system comprises a lens unit having the positive refractive power on the object side (the front lens part) and is different in power distribution from the zoom lens system according to the present invention. Further, the zoom lens system disclosed by U.S. Pat. No. 4,099,845 is different also in moving loci of the lens unit for zooming from the zoom lens system according to the present invention and has a low vari-focal ratio.
Furthermore, each of the zoom lens systems disclosed by U.S. Pat. No. 4,759,617 and U.S. Pat. No. 4,516,839 comprises a first lens unit which is movable for zooming and allows total length thereof to be varied by zooming, thereby making it difficult to make the lens barrel thereof waterproof.
Moreover, the zoom lens system disclosed by U.S. Pat. No. 3,549,235 consists of an attachment lens system composed of a first lens unit which is kept fixed for varying focal length and has positive refractive power, a second lens unit which is movable for varying focal length and has negative refractive power, a third lens unit which is movable for varying focal length and has negative refractive power and a fourth lens unit which is kept fixed for varying focal length and has positive refractive power, and a basic objective composed of a fifth lens unit which is kept fixed for varying focal length and has positive refractive power. Accordingly, this zoom lens system is different from the zoom lens system according to the present invention in fundamental composition such as power distribution.
In addition, the zoom lens system disclosed by U.S. Pat. No. 3,682,534 uses a third lens unit which is moved toward the image side for zooming from a wide position to a tele position in the direction opposite to the moving direction of a second lens unit and a third lens unit disposed therein, unlike a third lens unit adopted for the zoom lens system according to the present invention. Consequently, the zoom lens system disclosed by U.S. Pat. No. 3,682,534 has a high vari-focal ratio, but requires a relatively complicated moving mechanism for variation of focal length, thereby making it difficult to make the zoom lens system compact especially when the moving mechanism is taken into consideration.